Update the render module of centos-art.sh script.
    
    - Previously, some of the most important module environment variables
      were defined in the ${MODULE_NAME}.conf.sh file. Using the
      ${MODULE_NAME}.conf.sh file was discontinued a few days ago and
      variables inside them were undefined making the render module to
      fail. This update changes ${MODULE_NAME}.sh files to initialize
      module environment variables in them making the render module to
      work as expected.
    
    - Previously, we were using tcar_setModuleEnvironment function to load
      both modules and submodules inside centos-art.sh script with any
      different. Now, because we want to load first-level modules from
      sub-modules, we use tcar_setModuleEnvironment to load first-level
      modules only and tcar_setSubModuleEnvironment to load sub-modules at
      different levels only. This update changes the files inside the render
      module to start using both tcar_setModuleEnvironment and
      tcar_setSubModuleEnvironment functions when loading first-level
      modules and sub-modules.
    
    - There was a typo in asciidoc_setXhtmlRendition.sh file that
      prevented the render module from producing HTML chunks correctly.
      This typo was fixed and render module produces HTML chunks as
      expected.
    
    - Inside render.sh file, rename variable from DIRECTORY to ARGUMENT.
    
    - Add symlink sub-module to handle symbolic links rendition.
    
    - Add render modules' documentation manual.